Certain electrical circuits require data transmission between electrical instruments that are electrically isolated from one another. One example is when electrical instruments are coupled to a bus loop. Because bus loops provide communication between electrical instruments in addition to power, a problem can exist if the signal encoding schemes between the electrical instrument and the bus loop are not substantially the same. Electrical instruments utilizing bus loops, such as a two-wire bus loop receive power and communicate through the bus loop using analog signals by controlling either the voltage or current draw. This first analog signal is then converted into a digital signal, processed, converted back into a second analog signal, and transmitted to another instrument or a host system. This method of communicating is adequate so long as the first and second signals are based on the same scale. Typically, in a two-wire bus loop, the instrument varies the current between approximately 4-20 mA, where 4 mA corresponds to a minimum value and 20 mA corresponds to a maximum value. A problem can arise however, if one of the electrical instruments is operating on a different current range, for example if the analog signal is limited to between approximately 12-20 mA. Using this current range, 12 mA would correspond to a minimum value and 20 mA would correspond to a maximum value. An error may occur if the analog signal received from the electrical instrument operating on a 12-20 mA scale is sent to an electrical instrument operating on a 4-20 mA scale.
This error can be compounded in situations where the instruments are electrically isolated from one another. Although there are various configurations capable of such data transmission, one common configuration utilizes optically coupled circuits. Typically, one instrument of the optically coupled circuit generates a first analog data signal, which is converted to a digital signal using an analog-to-digital converter. The digital signal may comprise a serial bit stream value, which is transmitted using an optocoupler.
One problem with prior art optically coupled circuits is that they are limited in their ability to scale the first signal to accommodate instruments utilizing different signaling. In other words, the transmitted signal generally corresponds to the first analog signal encoding and not to the second analog signal encoding. This may be acceptable in limited situations; however, it may be desirable to scale the first signal to accommodate a different signal process. For example, if one of the instruments is optically coupled to a bus loop that operates on a scale different from the instrument itself, it may be necessary to scale the first signal to correspond to the second signal. The scaling may comprise any manner of linear or non-linear scaling to the signal such that the signal changes to accommodate the output analog signal encoding corresponding to another electrical instrument. Therefore, the prior art limits the first signals available to the instrument and therefore, limits the capabilities of the electrical instrument.
The present invention overcomes this and other problems and an advance in the art is achieved by performing runtime scaling of the bit-stream in order to provide an accurate second signal in situations where the first signaling does not substantially match the second signaling.